Chair back



FIG. 1 is a front elevation view of a chair back showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a bottom view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a right side elevation view thereof, the left side elevation view being a mirror image thereof; and,

FIG. 6 is a right side elevation view thereof.

The phantom lines in FIG. 6 are for illustrative purposes and form no part of the design as claimed. 

The ornamental design for a chair back, as shown and described. 